It is known to crack dicyclopentadiene to the monomer (U.S. Pat. No. 4,048,242, for example). It is also known to produce dihydroxycyclopentene (cyclopentenediol) by reacting the hydrocarbon with peroxyacetic acid followed by hydrolysis in cold water to produce the diol (Organic Synthesis, Coll. Vol. V., Pages 414-418). 2-cyclopentene-1-one is produced from this diol by treatment with a catalytic amount of a sulfonic acid, e.g., p-toluene sulfonic acid. (ibid. Pages 326-328). 2-alkylcyclopentadienes are also conveniently prepared by alkylation of cyclopentadienide salts of alkali metals or halomagnesium.
Cyclopenteneone is not useful to produce methylcyclopentenolone (synthetic maple flavor). 2-Methyl-2-cyclopenten-1-one is a useful starting material for the production of steroids (Synthesis (1980) pages 118-119). This recent reference lists 18 methods of making 2-methyl-2-cyclopenten-1-one ranging in yield from 16% to 90%. The authors recommend reduction of 3-isobutyloxy-2-methyl-2-cyclopenten-1-one derived from 2-methyl cyclopentane-1,3-dione with lithium aluminum hydride or diisobutylaluminum hydride followed by stirring in a mixture of ether and MnO.sub.2 to obtain pure 2-methyl-2-cyclopenten-1-one in a 62% yield, as being "the most convenient to perform in the laboratory, at least as efficient as other methods, and it adds another practical variant to currently known synthetic approaches to this compound."
The present invention provides an efficient, commercially suitable method for making a methyl product of somewhat lower purity (a mixture of the 2- and 3-isomers) in a combined yield of about 85% (78% 2-methyl-2-cyclopenten-1-one).
It has been found that the alkyl substituent has a profound effect on the relative reactivity of the double bonds in the alkylcyclopentadiene. The double bond adjacent the alkyl substituent is more highly reactive to peracids producing therefrom larger portions of the epoxides (a) and (b) below. This effect is reflected in the ultimate ratio of 2- and 3- alkylcyclopentenones. A surprisingly high selectivity in this respect was obtained.